1. Field of the Invention
The present invention relates to video display devices, and more particularly, to amplifiers and cut-off control circuits for adjusting the white balance of display devices.
2. Description of the Related Art
In a display device such as a monitor, a white balance adjustment makes a white object appear white regardless of the color temperature. In particular, in a cathode ray tube (CRT), a white balance adjustment adjusts gains and biases of signals applied to red, green, and blue (RGB) guns or cathodes in the CRT. The bias adjustment is often referred to as a cut-off adjustment. Cut-off control circuits can employ a DC-coupling or an AC-coupling when driving cathodes of a CRT. The coupling mode of the cut-off control circuit does not affect the gain adjustment since the gain controls an AC signal component that both AC and DC couplings transfer. However, the coupling mode does affect the bias adjustment, and cut-off control circuits typically require different integrated circuit chips according to the type coupling employed. A further concern is that brightness control depends on a combined RGB signal, but the cut-off control, which controls cathode biases, is carried out for each of the R, G, and B signals separately. In order to adjust the white balance, the brightness level is adjusted first, and then the cut-off control is performed.
FIG. 1A is a circuit diagram of a conventional cut-off control circuit having a DC-coupling to the video portion of a monitor (e.g., a cathode in a CRT). The cut-off control circuit of FIG. 1A includes a video pre-amplifier 101 and a drive amplifier 102 for driving a CRT. For simplicity, FIG. 1A shows only one channel even though a color video system normally has three channels (R, G, and B). The cut-off control adjusts the respective CRT cathode bias voltages VA for the R, G, and B cathodes. In FIG. 1A, a video feedback voltage Vf provides a negative feedback to pre-amplifier 101. Pre-amplifier 101 controls an output voltage Voutput so that video feedback voltage Vf is the same as or depends on a brightness control voltage Vbright. A current controller 103 controls video feedback voltage Vf and thus controls voltage Voutput and CRT cathode bias voltage VA. Equation 1 expresses CRT cathode bias voltage VA as a function of feedback voltage Vf, resistances R1 and R2 of respective resistors 104 and 105, and a current Ic. ##EQU1##
As can be seen in Equation 1, the CRT cathode bias voltage VA depends on the control current Ic, which a cut-off control signal Vcutoff can adjust.
FIG. 1B is a circuit diagram of the conventional video pre-amplifier 101 shown in FIG. 1A, in which an external feedback brightness control method is used. Referring to FIG. 1B, the video pre-amplifier 101 includes an adder 110a, a drive amplifier 101b, a comparator 101d, and a switch 101c.
FIG. 2A is a circuit diagram of a conventional cut-off control circuit using an AC-coupling mode. The cut-off control circuit of FIG. 2A includes a video pre-amplifier 201, a drive amplifier 202, a coupling capacitor 203, and a comparator 204. This circuit uses a separate DC bias circuit that controls cathode bias voltage VA. Only an AC component of the output signal from drive amplifier 202 passes through a coupling capacitor 203 to voltage VA. Equation 2 expresses cathode bias voltage VA as a function of a reference voltage Vref, resistances R3, R4, and R5 of respective resistors 213, 214, and 215, and cutoff voltage Vcutoff. ##EQU2##
As shown in the equation 2, cathode bias voltage VA depends on cut-off control voltage Vcutoff which is applied to a comparator 204 to adjusts a control current Ic.
FIG. 2B is a circuit diagram of video preamplifier 201 shown in FIG. 2A, in which a built-in feedback brightness controlling method is used. The video pre-amplifier 201 of FIG. 2B includes an adder 201a, a drive amplifier 201b, a comparator 201d, and a switch 201c.
As mentioned above, the video pre-amplifiers 101 and 201 of the conventional cut-off control circuits respectively using DC-coupling and AC-coupling differ from each other. Thus, conventional cut-off control circuits using different coupling modes require different integrated circuits and external parts. Further, the cut-off control circuits require a number of external parts, such as an operational amplifier and resistors, in addition to the video pre-amplifier. This makes the circuits more complex.